Calibrated vapor generator source

ABSTRACT

A portable vapor generator is disclosed that can provide a controlled source of chemical vapors, such as, narcotic or explosive vapors. This source can be used to test and calibrate various types of vapor detection systems by providing a known amount of vapors to the system. The vapor generator is calibrated using a reference ion mobility spectrometer. A method of providing this vapor is described, as follows: explosive or narcotic is deposited on quartz wool, placed in a chamber that can be heated or cooled (depending on the vapor pressure of the material) to control the concentration of vapors in the reservoir. A controlled flow of air is pulsed over the quartz wool releasing a preset quantity of vapors at the outlet.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. DE-AC07-76ID01570 between the U.S. Department of Energy andEG&G Idaho, Inc.

FIELD OF THE INVENTION

This apparatus and method will provide a controlled source of vapors totest and calibrate various types of vapor detection systems. A sourcesubstance is coated on quartz wool and contained in a reservoir. Thereservoir is heated or cooled (depending on the vapor pressure of thesource) with a controlled thermoelectric heater/cooler releasing aquantity of substance molecules into a pulsed air stream which can becalibrated with a spectrometer or other analytical instrument.

BACKGROUND OF THE INVENTION

There is an increasing need for detection of very low levelconcentrations of narcotic and explosive vapors. Security personnel inairports and other sensitive areas are currently using apparatus thatsense these vapors in the parts per trillion (ppt) range. At the presentwriting, the National Institute for Standards and Technology does nothave standards for explosive vapors; therefore, a portable calibratedvapor generator source is critically needed in the field to test andcalibrate those vapor detection devices such as that disclosed in U.S.Pat. No. 5,157,261 issued Oct. 20, 1992, which uses fiber opticspectroscopy and changes in fluorescence to detect explosives. U.S. Pat.No. 4,820,920 issued Apr. 11, 1989, discloses a second method andapparatus for detecting explosive or illegal drugs by microwave or RFradiation and then injecting the substance into a mass spectrometer forspectrum analysis.

It is desirable to be able to test and calibrate these type devices inthe field to determine operability and accuracy at low concentrations.It is therefore the purpose of this invention to describe an apparatusand method for calibrating these detection devices in the field using aportable calibrated vapor generator source.

SUMMARY OF THE INVENTION

The invention generally stated is a calibrated vapor generator apparatuscomprising:

a pressurized clean air supply;

means for controlling the flow of pressurized clean air in communicationwith the clean air supply;

means for controlling a pulse time for the flow of pressurized air incommunication with the means for controlling the flow of the pressurizedclean air;

a reservoir means for desorbing a known quantity of a vapor sourcesubstance in communication with the means for controlling the pulsetime;

means for sensing and indicating the quantity of vapor source substancedesorbed from the reservoir means by an electronic integrator/controllerthereby providing a calibrated pulse of vapor to a vapor detectiondevice.

Additionally, this invention discloses a method of using the apparatusfor providing a controlled and known source of vapors to test andcalibrate vapor detection systems, comprising the steps of:

coating a carrier with a known quantity of a vapor source substance;

placing the carrier within a reservoir;

controlling a temperature within the reservoir;

passing a pressure controlled clean air pulse through the reservoir;

measuring the pressure and duration time of the pulse;

integrating the pressure as a function of time of the pulse; and then

indicating a weight of the vapor source substance, thereby providing aknown quantity to test and calibrate the vapor detection system.

Other objects, advantages, and capabilities of the present inventionwill become more apparent as the description proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood and further advantages and usesthereof may become more readily apparent when considered in view of thefollowing detailed description of the exemplary embodiments, taken withthe accompanied drawings, in which:

FIG. 1 is a piping schematic drawing of the vapor generator;

FIG. 2 is a perspective view of the portable vapor generator case;

FIG. 3 is a front view of the generator control panel;

FIG. 4 is an electrical and piping block diagram;

FIG. 5 is a diagramic view of the hand-held vapor generator head;

FIG. 6A is a plan view of the quartz wool reservoir and attachedthermoelectric heater/cooler;

FIG. 6B is a side view of the reservoir taken through lines 6B of FIG.6A;

FIG. 7 is a typical TNT spectra from a calibration check using an ionmobility spectrometer (IMS);

FIG. 8 is a typical RDX spectra using the IMS; and

FIG. 9 is a typical PETN spectra using the IMS.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the vapor generator apparatus 10 is shown inschematic form. The clean source of air is provided by pump 12 and pipedto filter assembly 13 consisting of a molecular sieve 14 which is amoisture filter and a first activated charcoal filter 16 to removehydrocarbons. The pressure regulator 18 and flow control valve 20maintain specific air flows by carefully controlling the pressure. Theair flow is first set up by passing air through a flexible 5' tubing 22and out vent port A of 3-way solenoid valve 24. The flow is properly setthrough port A by set-flow orifice 25. The solenoid valve 24 thendirects air to port B, a pressure transducer 26, a second activatedcharcoal filter 28, and into the quartz wool reservoir 30. It is withinthe reservoir that the source vapor is desorbed from the quartz woolinto the passing air stream, exiting through exit heater 32 into anadjacent vapor detector 34 (shown in phantom), which is beingcalibrated. The amount of vapors exiting the generator 34 is controlledby a pair of thermoelectric heater/coolers 36 and the length of time andpressure in the flow-control means 38, i.e., the integral of pressureover time. This is presented in the following formula: ##EQU1## Wherethe weight in picograms of gas is proportional to the integral ofdifferential pressure over time. One heater/cooler is on the front ofreservoir 30 as shown, the other on the back. This time T is controlledby completing a pulse of air flow when closing 3-way valve 24 port C andopening 3-way vent valve 40 to port C.

The thermoelectric heater/coolers 36 operate to heat or cool thereservoir 30 depending on which chemical source is on the quartz wool,e.g. TNT would require a temperature of about 20° C., RDX a temperatureof about 70° C., and PETN a temperature of about 65° C.

In this apparatus, the air pump 12, sieve 14, filter 16, pressureregulator, and flow controller are contained within case 42 (shown inphantom) and the remaining components are contained within a "head" 90as will be described later. The umbilical connection being made byTeflon™ tube 22.

Referring now to FIG. 2, one can see some of the internal componentswithin case 42. In addition to the air pump 12, sieve 14, filter 16,pressure regulator 18, and flow control valve 20, there are electricaldevices consisting of thermoelectric controller (TEC) 44 that controlsthe thermoelectric heater/coolers, electronic integrator/controller 46,and an electric cooling fan 48 that cools the air compressor 12 andelectrical devices. The controls for these various components arelocated on front cover control panel 50 and will be described byreferring to FIG. 3.

FIG. 3 illustrates the details of control panel 50. From right to left,the devices are: a power on/off push button switch 52 and fuse 54, asource weight digital readout indicator 56, a pulse indicator light 58and ready indicator light 50, a set point control (potentiometer) 62, asetup/run control switch 64 and setup indicator light 66 and runindicator light 68, an arm/disarm switch 70 and arm indicator light 72and disarm indicator light 74, an air pressure gage 76, an air quickdisconnect coupling 78, and a vapor head (wand) electrical plug 80.

FIG. 4 is an electrical and piping block diagram illustrating thecomponents contained within case 50. Incoming AC power 82 is connectedto a power supply 84 electronic integrator/controller 46, thermoelectriccontroller 44, and air pump 12. The power supply 84 provides power tothe solenoid valve controller 86, the heater controller 88, and fan 48.Electrical outputs (and some inputs) connect between the 25-pin electricplug 80 and the thermoelectric controller 44, the valve controller 86,the heater controller 88, and the integrator/controller 46. The air pump12 and air flow control means 38 discharge clean air to the airquick-disconnect coupling 78.

FIG. 5 discloses the components within the interchangeable head 90 whichconnects by electric cable 91 and flexible tubing 22 to the case. Thereare also quick disconnects for the cable at 92 and tube disconnect at 94similar to quick disconnect coupling 78 and plug 80 (FIG. 3). 0n orwithin the head 90 is: the 3-way solenoid valve 24 and connectingorifice 25, the 3-way vent valve 40, pressure transducer 26, and secondcharcoal filter 28. The reservoir 30 and attached heater/cooler 36 areconnected to the filter 28 by a reservoir entrance fitting 108 and anINITIATE push button 95 that starts the air is located on the front ofthe head 90. A pair of LED lights 93 for READY and PULSE are on top ofthe head.

The details of the reservoir 30 can be seen in FIG. 6A, which is shownwith the thermoelectric heater/cooler removed. The body of the reservoir30 has been drilled to provide a series of six connecting apertures 96(in phantom) which contain the substance coated quartz wool 98, i.e.,TNT, RDX, PETN, or narcotics.

Thermistor 100 controls the temperature of the thermoelectric heaters 36(FIG. 6B) and the exit heater 32, respectively. The exit heater 32maintains the exit temperature about 2° above the reservoir to preventthe vapors from coating the exit tube. A 0.5 micron stainless steel frit104 (composite filter) removes any particles from the air stream above0.5 micron from the reservoir apertures 96. The carrier or quartz wool90 is inserted in the apertures 96 and then coated with a methyl ethylketone (MEK) solvent containing the explosive (or methanol(MEOH) solventcontaining a narcotic). The MEK or (MEOH) evaporates leaving theexplosive or narcotic coating on the wool 98. Then, the end caps 106 andentrance fitting 108 are threaded into the reservoir 30.

FIG. 6B illustrates the thermoelectric heater/cooler 36 mounted on thereservoir 30.

In operation, the air pulse from the interchangeable head 90 (FIG. 5)enters at the entrance fitting 108 passes over the quartz wool 98 andexits the heater 32 into the vapor detector 34 that is being calibrated.

The overall apparatus operation will be described by referring to FIGS.1, 2, and 3. It is assumed that the cables 91 and tube 22 are connectedto the case 42 and head 90.

The system is activated by setting the arm/disarm switch 70 to DISARMand pushing the power ON button 52, which starts air pump 12, energizesthe thermoelectric controller 44, thermoelectric heater/cooler 36, andexit heater 32. Check or set the temperature set point on thethermoelectric controller 94 to the desired ohms obtained from thethermistor resistance chart. Set the air pressure regulator 18 to127+7-O K Pa (18+1-O psi). After a 10-minute warmup, set the run/setupswitch 64 to RUN to check that the weight (picogram) meter 56 readszero. Adjust the set screw above the meter as necessary and then set therun/set switch 64 to SETUP. Turn the set point potentiometer 62 to thedesired setting as read on the picogram meter 56. Set the run/setupswitch 64 to RUN and the arm/disarm switch to ARM. Check the READY LEDlights, i.e., 60 on FIG. 3, and 93 on FIG. 5. Start the pulse by pushingthe INITIATIVE push button 95 on the front of the head 90. The red PULSElight 93 on the head 90 and red PULSE light 58 on the control panel 50will come on during the pulse. Readout the digital number on thepicogram weight indicator 56.

Wait for the READY lights to come on before another INITIATE cycle.

The device being tested can now be readout or its alarm set pointverified by comparing it to the picogram indicator 56. The picogramweight can also be mathematically converted to other units of measure,e.g., parts per trillion (ppt).

The technical specifications for the apparatus are as follows:calibrated to a reference instrument; picogram to nanogram output;variable temperature from 20°-80° C.; source temperature control ±0.1°C.; flow range 0-300 ccm; variable pulse width; interchangeablehand-held portable head, i.e., a different head for different explosivesor narcotics; digital readout of mass outputs; size of case: 18inches×13 inches×8.5 inches; weight of case: 33 pounds; and power: 110volt AC.

FIGS. 7, 8, and 9 illustrate the results of calibration of earlierlaboratory experimental vapor generator using three explosive vapors.Explosive mixtures can be made in the form of plastic explosives, whichare made of an explosive chemical usually bound in a polymer matrix.Their main advantage is that they can be molded or cast into any desiredshape or size. The explosive chemical is typically cyclonite (RDX),pentaerythritol tetranitrate (PETN), and trinitrotoluene (TNT) whichhave been used in this experimental lab setup. The machine used tocalibrate the vapor generator is called an ion mobility spectrometer(IMS) which records the amount of time that it takes for an ion totravel to an electrified plate through a specific vapor, where the ionand vapor collisions slow down the ion mobility. The graphs of FIGS. 7,8, and 9 show the millivolt output of the spectrometer versus time inmilliseconds for the three explosives: TNT, RDX, and PETN, respectively.The three peaks 110, 112, and 114 are at 14.3 ms, 15.8 ms, and 18.8 ms,respectively. The retention times correspond to a specific IMS (PCP IMS110) and specific conditions of 160° temperature and 646 torratmospheric pressure.

The output from the IMS is monitored in a specified time window(typically 550 μs wide) as at 116, centered on the peak associated withthe explosive to be quantified. The voltage in this window is integratedby an integrator and then subtracted from the integrated voltage in abackground window. The background window is set close to the signalwindow in a region that is clear of extraneous peaks. This delta voltageis next sent to a second integrator. The integrator integrates theoutput voltage versus time. By integrating the area between the lines(about 550 nanoseconds on either side of the peaks) as at 116, theamount of substance can be accurately determined.

While a preferred embodiment of the invention has been disclosed,various modes of carrying out the principles disclosed herein arecontemplated as being within the scope of the following claims.Therefore, it is understood that the scope of the invention is not to belimited except as otherwise set forth in the claims.

What is claimed is:
 1. A calibrated vapor generator apparatuscomprising:a. a pressurized clean air supply; b. means for controllingthe flow of pressurized clean air in communication with the clean airsupply; c. means for controlling a pulse time for the flow ofpressurized air in communication with the means for controlling the flowof the pressurized clean air; d. a reservoir means for desorbing a knownquantity of a vapor source substance in communication with the means forcontrolling the pulse time; e. means for sensing and indicating thequantity of vapor source substance desorbed from the reservoir, saidmeans further comprising a pressure transducer and an integratingcircuit wherein the weight of the substance is proportional to theintegral of air pressure over time and is indicated on an electronicreadout device.
 2. The apparatus as recited in claim 1 wherein thepressurized clean air supply comprises an air pump and connecting filterassembly.
 3. The apparatus as recited in claim 1 wherein the means forcontrolling the flow of pressurized air comprises a pressure regulatorand connecting flow control valve.
 4. The apparatus as recited in claim1 wherein the means for controlling the pulse time comprises a pair of3-way solenoid valves in serial connection contained within aninterchangeable head.
 5. The apparatus as recited in claim 4 wherein thereservoir means is within the interchangeable head and comprises atemperature controlled body having a plurality of serial connectedapertures containing a coating on quartz wool, said coating being thevapor source substance.
 6. The apparatus as recited in claim 5 whereinthe temperature of the body is controlled by a pair of thermoelectricheater/coolers affixed on opposing sides of the body and is controlledwithin 0.1° C.
 7. The apparatus as recited in claim 1 wherein the vaporsource substance is TNT, RDX, or PETN.
 8. The apparatus as recited inclaim 1 wherein the vapor source substance is an illegal drug.
 9. Amethod for providing a controlled and known source of vapors to test andcalibrate vapor detection systems, comprising the steps of:a. coating acarrier with a known quantity of a vapor source substance; b. placingthe carrier within a reservoir; a controlling a temperature within thereservoir; d. passing a pressure controlled clean air pulse through thereservoir; e. measuring the pressure and pulse time; f. integrating thepressure as a function of time of the pulse; and then g. indicating aweight of the vapor source substance, thereby providing a known quantityto test and calibrate the vapor detection system.
 10. The method asrecited in claim 9 wherein the vapor source substance is TNT, RDX, orPETN and the carrier is a quartz wool.
 11. The method as recited inclaim 9 wherein the temperature within the reservoir is maintained by athermoelectric heater/cooler to within ±0.1° C.
 12. The method asrecited in claim 9 wherein integrating the pressure and pulse time isperformed by an electronic integrator/controller, a valve controller, apair of solenoid valves, and a pressure transducer.
 13. The method asrecited in claim 12 wherein the clean air pulse is provided by an airpump, a filter assembly, a pressure regulator, and a flow control valve,through a tubing to and an interchangeable head containing thereservoir, solenoid valves, and pressure transducer.